Various digital modulation methods are known in which the phase angle, the frequency, the amplitude of a carrier signal or combinations thereof are varied in a manner dependent on useful data to be transmitted.
In the case of the widespread mobile radio standard GSM, Global System for Mobile Communication, the modulation method used is GSMK, Gaussian Minimum Shift Keying. This involves a phase-continuous frequency shift keying method with a modulation index of 0.5, in which Gaussian pulses are used instead of rectangular pulses for the data.
In order to meet the demand for the transmission of ever higher volumes of data, combined phase and amplitude keyings are increasingly being used instead of frequency shift keying methods. In the case of said combined phase and amplitude keyings, part of the signal information is contained in the signal amplitude. It is necessary in this case to use amplifiers that satisfy high linearity requirements. However, the gain of these amplifiers has to be regulated in a manner dependent on the received field strength in order to provide for constant amplitude conditions at the receiver output in order that the data can be demodulated optimally.
In the various mobile radio methods, the received field strength usually depends on the chosen carrier frequency. One reason for this is due to the frequency-dependent channel properties during signal transmission.
At the present time, signal amplifiers with a programmable gain, a so-called PGC, programmable gain control, are normally used in radio receivers for modulation methods which use a phase and amplitude shift keying.
In widespread mobile radio standards, such as GSM for example, which organize the data transmission in so-called time slots, the reception gain has hitherto been set in a manner dependent on an evaluation of the reception signal in the preceding time slot. The signal received in the current time slot is accordingly disadvantageously received with the optimum gain setting applicable to the preceding time slot.
However, some radio systems that operate with time slots use a frequency hopping method, that is to say that transmission is effected at a different frequency in each time slot according to a previously known pattern. On account of the frequency-dependent channel properties of the radio signal, however, this disadvantageously results in different received field strengths in each time slot.
Consequently, an algorithm for gain setting that operates as described and uses the signal amplitude of the preceding time slot as a basis for the gain setting makes an error, in principle. This error has to be compensated for by a higher dynamic range at the output of the amplifier and a larger bandwidth of the analog/digital (A/D) converter connected downstream.
One possibility for realizing a correction of a fluctuating received field strength in the radio receiver is automatic gain control, so-called AGC, as was used earlier in AM broadcast radio receivers. However, this would have the disadvantage that the amplitude information of the useful signal would be corrupted since the AGC corrects slow amplitude fluctuations that may be present in the useful signal.
The document DE 1 466 163 specifies an automatically tuned radio frequency circuit with an arrangement for automatic gain control. In this case, the intention is to automatically control the gain in a transmission channel without using mechanical devices. A binary-gradated attenuation device driven by a multistage binary register switch is shown for this purpose. The multistage binary register switch is driven by a multivibrator. A comparator connected between an amplifier connected downstream of the attenuation device and the multivibrator is furthermore provided.